Modern integrated process flows typically may have many patterning and implantation steps to form the variety of transistors in an IC. For example, core transistors with low turn on voltage (vt), nominal vt, and high vt may be formed in addition to input/output (I/O) transistors and memory cell transistors such as SRAM transistors. Each transistor type typically requires a vt pattern and implant, gate doping pattern and implant, a source and drain extension pattern and implant, and a deep source and drain pattern and implant. A CMOS process flow typically builds both a pmos and nmos transistor of each transistor type. A CMOS flow with 3 core transistor types plus SRAM transistors and I/O transistors may have 20 or more patterning and implant steps to form 5 different nmos transistors and 5 different pmos transistors. In addition if other embedded devices, such as resistors, capacitors or bipolar transistors are constructed, additional patterns and implant steps may be added.
Electronic digital and analog integrated circuits often require resistors to perform the desired functions. One type of resistor which may be formed on an integrated circuit is a polysilicon resistor. N-type or p-type polysilicon resistors are typically formed using source/drain ion implantation or gate doping ion implantation.
One important property of embedded resistors, especially for analog circuits, is the temperature coefficient of resistance (TCR) which measures the change in resistance with a change in temperature. To reach sufficiently low TCR, the impurity of doping concentration must be very high, about 3×1020/cm3 for polysilicon resistors. Typically, additional implants are performed in an integrated process flow with embedded polysilicon resistors to provide resistors with low TCR. These additional implants usually require one or more additional photomask levels and add cost and cycle time to the integrated circuit process flow.